I can't answer your questions so I suggest checking out the presentation
itself when it appears (Hans Mellberg wrote: "We expect the paper to be
posted around next Monday or so, at www.scvemc.org. More work has been done
to this presentation which will show up at the August EMC Symposium.")

However, the simulations clearly showed that under the conditions of
excitation studied there was more radiation predicted from the 20H-type
edge. The quick explanation was that the 20H edge presents a gentler
transition to the impedance of free space, which is higher than that of the
medium supporting the wave between the planes. This better match results in
a higher transmission coefficient than the simple-edge case, meaning more
energy emitted and less reflected back into the board.

Thanks for the nice summarization. Please allow me to raise some more
questions.

The intention of 20H rule is to reduce the radiation from the space between
pwr plane and gnd plane. However, "The simulations presented showed that 20H
structures actually resulted in more emission at the board edge." Did
anybody measure the radiation from the board edge to validate the 20H rule
yet?

I would do my best to reduce common mode voltage in pwr/gnd planes when
designing PCB, and pay less attention to differential mode voltage between
pwr and gnd planes. Does the radiation from board edge come from CM or DM
voltage? If DM, does it make main contribution to the radiation of the whole
board?

Here is what I think I heard at the EMC society presentation. The
simulations presented showed that 20H structures they examined actually
resulted in more emission at the board edge. However, the presenters were
neutral on whether this was detrimental to compliance for the system as a
whole, pointing out that EMC solutions are rarely universally applicable. I
hope they post this on the web, I found it most instructive.

The main points I remember:

1) Time-varying currents on vias can inject radial TEM-mode waves into the
space between planes.

2) The energy thus injected bounces around the cavity volume between the
planes. The board edge is a discontinuity in the medium and so results in
partial reflection of the propagating wave and partial transmission, i.e.
radiation from the board edge.

3) Fencing the board edge with grounded vias is equivalent to changing the
PCB-edge discontinuity to a short to ground, so the reflection coefficient
becomes -1 and all energy is kept inside of the fenced area.

4) By contrast, a 20H-rule example showed that that structure, which looks a
little like a patch antenna, allows for more efficient radiation from the
edge. The exposed area allows a propagation mode where energy can travel
around the outside edges of the board also. Thus less energy is trapped
within the board area and more gets radiated.

5) Is this good? Energy bouncing around between planes can be picked up by
structures like the one that initially injected it, e.g. vias, and then
travel along conductors to outside surface components where it can be
emitted. This is not especially desirable. On the other hand, the more
efficient radiatiing edge (20H) puts more energy into the system chassis,
which moves the problem one level higher.

6) Closely spaced ground vias all across the board had the effect of fencing
in the injected energy to a small area. This seems to cut radiation from the
edges drastically. I would like to know more about this particular case.

The data was obtained from an FDTD simulation of a small board with a single
off-center via as the point of injection. Excitation was with both a
continuous sine-wave at 1GHz and also a Gaussian-derivative pulse.
Dielectric losses were included in the FR4 model. The results of lengthy
simulations were presented as captivating animations, with color variation
showing the magnitude of the Poynting vector across the whole board. The
test setup was of necessity rather artificial but it did help to give a feel
for the physical behavior underlying radiation from board edges.

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